One proposed structure of a signal transmission device has a base plate with an internally formed optical waveguide. A light emitting element, such as a laser diode, of inputting light into the optical waveguide is placed in a recess formed in the direction of depth of the base plate from its surface (see, for example, Patent Document 1). In the signal transmission device of this prior art structure, an electronic part with a driving circuit for driving the light emitting element is mounted on a rear face of the base plate. This arrangement aims to shorten the wiring length of an electrical connection between the driving circuit of the electronic part and the light emitting element placed in the recess and thereby reduce a response delay of the light emitting element.    Patent Document 1: Japanese Patent Laid-Open No. 2001-174657 (FIG. 6)